The interactions of the oral actinomyces and Candida albicans with polymorphonuclear leukocytes (PMNs) and the complement system have been further defined. Actinomyces viscosus T14V and A. naeslundii WVU45 are destroyed by human polymorphonuclear leukocytes. Electron microscopy indicates that this process is attributable to phagocytosis. The lectin associated with the type 2 fimbriae of the actinomyces initiates the bactericidal activity by binding to carbohydrate containing receptors on the PMNs following exposure of these receptors by sialidase, an enzyme produced by the actinomyces. Mutants of A. viscosus T14V and A. naeslundii WVU45 lacking type 2 fimbriae are not phagocytosed. Bactericidal activity is inhibited by lactose and Beta-methylgalactoside but not by cellobiose or Alpha-methylgalactoside. Antibodies reactive with the actinomyces type 1 or type 2 fimbriae as well as the cell wall stimulate phagocytosis in the absence of neuraminidase. Two antisera, one raised against the parent strain of A. viscosus T14V and one against the mutant lacking fimbriae are dependent on complement activation with the resultant deposition of the C3b/C3bi fragments of the third component of complement (C3) for optimal phagocytosis. Receptors for C3bi and a further degradation product of C3 (C3d) are present on the hyphal form of Candida albicans. This receptor, which has been identified by rosetting with C3d coated erythrocytes, is a mannosylated protein and has been partially purified by ion exchange and C3d affinity chromatography. One major and two minor bands are present in the acid eluate from the affinity column. Candida carrying C3bi or C3d may attach to the complementary receptors on other organisms with the resultant enhancement of the yeast population at a specific site. Studies concerning the cooperative effects of monoclonal antibodies in complement activation to define the relationship between the epitopes of the type 1 or the type 2 fimbriae of actinomyces are being extended. The distances between these epitopes are being measured by energy transfer between pairs of fluorophore conjugated Fab fragments of the monoclonal antibodies.